How to access number in Enum Property?
What I would do is design the items of Enum property like this:

items = [('0','No Smooth',''), 
         ('1','Smooth',''),
         ('2','Smoother',''),
         ('4','Smoothest','')]


Then you just need to cast it to int.

bpy.context.object.modifiers["Subsurf"].render_levels = int(self.SubSurfEnum )


When you change this Enum property and the subsurf levels you don't have to change any dictionary.